This past weekend I drove back home from Houston which is a 1500 mile trip. I kept very close track of my MPG all along the way just out of curiosity and also because I was disappointed in the highway MPG I was getting in Houston this summer. Being from Pennsylvania and with the 3.21 gears I was used to low to mid 20s mpg on the highway, but all I could get all summer long in Houston was 19 mpg with purely highway commuting. I chalked it up to fast driving (75 - 80 mph) among other factors.

But on the way back I noticed something very interesting. As soon as the outside ambient air temperature (thus also probably the intake air temp at highway speeds) dropped below about 90 degrees F, my highway fuel economy -immediately- jumped back to what I was used to (22 - 23 mpg at 70 - 75 mph highway). Also most factors (other than the air temp) are accounted for with this because I just had the cruise control set in 6th gear at 72 - 75 mph the entire way back as well as I ran the air conditioning the entire way back. All the way from Houston to Nashville I got a steady 19 mpg. Once we crossed through Nashville, the air temps dropped below 90 and from there on I got 22 - 23 mpg with everything else the same. I was struck by this because I started driving through the mountains after Nashville so I was expecting to see worse fuel economy but it went the complete opposite direction.

I am fairly certain (maybe RIPP has some insight on this from tuning the JK fuel map) that at high air temps the factory map probably runs rich (in addition to retarding the spark a little) to prevent knock with regular 87 octane gas when it is running in hot conditions. One thing I would maybe suggest to people living in the southern states is to get a few extra MPG give higher octane gas a try. I actually noticed on the allpar website that the 3.8 was recommended to use 89 octane when it was in the caravan, but 87 was "OK" to use (probably because knock sensors dial back the spark if it senses knock). If I had noticed this before leaving Houston I would have experimented with running 89 and 93 octane gas for a tank or two in the high air temps to see whether paying an extra 3 - 5% for gas you could get an extra 5 - 10% mpg and some extra power (saving money and more than likely keeping higher power output numbers...a win-win).

 

That is interesting. I'm wondering if the mpg computer is reliable as well? Have you checked mileage manually to corroborate the onboard computer? I've only used 87 octane, but I think I will try higher octanes to see if its improved for my hot climate.

 

I am currently checking this tank of gas to see if the onboard computer is accurate, but the computer has been showing 17.6 running 87. I switched this tank to 89, and the computer is showing 18.4. I have to fill up today, so I will manually check it when I do.

 

I've noticed it also in Florida Some of it has to do with density altitude. The hot humid air at sea level is like being above 2000 feet. The engine isn't as efficient as it is with nice cool air. There may be other factors along with that: tire pressure, driving conditions, etc.

 

I'll try that as well! Good observation!

 

I'm interested now bc of what 1st timer said.... I have an 09 4dr Rubi with the 6 speed and I'll be doing good to get 17 on the interstate and 14-15 around town. I run 87 tho so I think I'll try 89 next fill up and see if that makes a difference......

 

The thing that amazed me was that all my calculations were by filling up and resetting the tripometer for each tank BUT the dash read-out was fairly accurate. When I was averaging 19 mpg the dash read-out was showing about 19.x mpg. When I noticed the jump to 22 - 23 mpg or so on the first tank I checked what the dash said expecting it to still be reading 19.x mpg, but it had increased to about 21.x mpg or so. So it seems like the dash is fairly accurate when all you do is drive highway miles. I think the dash loses accuracy when you drive in the city.

 

None of this is really surprising to me.

Too hot and you get bad gas mileage
Too cold and you get bad gas mileage
Too high altitude and you get bad gas mileage
Too low altitude and you get bad gas mileage

The computer can adjust to keep the jeep running but it can only work with the fuel and air it is receiving, it may keep running but may create less power per gallon of fuel causing you to have to rev more causing lower fuel economy the computer is constantly performing a balancing act to try to get the best fuel economy given the conditions.

 

As a pilot, I'm acutely aware of "density altitude" and it's effect on engine performance since they beat it into you during training. I can tell you any combustion engine is susceptible to it, and would make a difference.

 

Did the gas formulation change any? Ethanol in the gas will cut mileage noticeably.